SEMICONDUCTOR GAS SENSORS This page intentionally left blank Woodhead Publishing Series in Electronic and Optical Materials
SEMICONDUCTOR GAS SENSORS
Second Edition
Edited by RAIVO JAANISO University of Tartu, Tartu, Estonia OOI KIANG TAN Nanyang Technological University, Singapore Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom
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Typeset by TNQ Technologies Contents
Contributors xi
Part One Basics
1. Fundamentals of semiconductor gas sensors 3 Noboru Yamazoe and Kengo Shimanoe 1.1 Introduction 4 1.2 Classification of semiconductor gas sensors 5 1.3 Resistor-type sensors: empirical aspects 6 1.4 Resistor-type sensors: theoretical aspects 14 1.5 Future trends 34 References 37
2. Conduction mechanism in semiconducting metal oxide sensing films: impact on transduction 39 N. B^arsan, M. Huebner and U. Weimar 2.1 Introduction 39 2.2 General discussion about sensing with semiconducting metal oxide gas sensors 41 2.3 Sensing and transduction for p- and n-type semiconducting metal oxides 47 2.4 Investigation of the conduction mechanism in semiconducting metal oxide sensing layers: studies in working conditions 57
2.5 Conduction mechanism switch for n-type SnO2–based sensors during operation in application-relevant conditions 66 2.6 Conclusion and future trends 67 References 67
3. The effect of electrode-oxide interfaces in gas sensor operation 71 Sung Pil Lee and Chowdhury Shaestagir 3.1 Introduction 72 3.2 Electrode materials for semiconductor gas sensors 74 3.3 Electrode-oxide semiconductor interfaces 95 3.4 Charge carrier transport in the electrode-oxide semiconductor interfaces 104
v j vi Contents
3.5 Gas/solid interactions in the electrode-oxide semiconductor interfaces 119 3.6 Conclusions 124 References 125
4. Introduction to semiconductor gas sensors: a block scheme description 133 Arnaldo D’Amico and Corrado Di Natale
4.1 Introduction 133 4.2 The sensor blocks 135 4.3 Metal oxide semiconductor capacitor: the case of the
hydrogen gas sensitivity of Pd-SiO2-Si 142 4.4 Light-addressable potentiometric sensor 144 4.5 Metal oxide semiconductor field-effect transistor 148 4.6 Metal oxide semiconductors 151 4.7 Conclusions 156 References 156
Part Two Materials
5. One- and two-dimensional metal oxide nanostructures for chemical sensing 161 E. Comini and D. Zappa 5.1 Introduction 161 5.2 Deposition techniques 162 5.3 Conductometric sensor 169 5.4 Transduction principles and related novel devices 170 5.5 Conclusion and future trends 174 References 175
6. Hybrid materials with carbon nanotubes for gas sensing 185 Thara Seesaard, Teerakiat Kerdcharoen and Chatchawal Wongchoosuk
6.1 Introduction 186 6.2 Synthesis of carbon nanotube 192 6.3 Preparation of carbon nanotubedmetal oxide sensing films 194 6.4 Sensor assembly 199 6.5 Characterization of carbon nanotube–metal oxide materials 200 6.6 Sensing mechanism of carbon nanotube–metal oxide gas sensors 205 Contents vii
6.7 Fabrication of electrodes and CNT/polymer nanocomposites for textile-based sensors 206 6.8 Sensor assembly for textile-based gas sensors 210 6.9 Characterization of CNT/polymer nanocomposites sensing materials on textile substrate 212 6.10 Sensing mechanism of CNT/polymer nanocomposites sensing materials on fabric substrate 215 6.11 Conclusion 216 Acknowledgments 217 References 217
7. Carbon nanomaterials functionalized with macrocyclic compounds for sensing vapors of aromatic VOCs 223 Pierrick Clément and Eduard Llobet
7.1 Introduction 223 7.2 Cyclodextrins 226 7.3 Calixarenes and derivatives 229 7.4 Deep cavitands 230 7.5 Conclusions 232 Acknowledgments 235 References 235
8. Luminescence probing of surface adsorption processes using InGaN/GaN nanowire heterostructure arrays 239 Konrad Maier, Andreas Helwig, Gerhard Muller€ and Martin Eickhoff 8.1 Adsorptiondkey to understanding semiconductor gas sensors 239 8.2 III-nitrides as an emerging semiconductor technology 243 8.3 Photoluminescent InGaN/GaN nanowire arrays 243 8.4 Optical probing of adsorption processes 245 8.5 Experimental observations of PL response 246 8.6 Analysis of adsorption phenomena 250 8.7 Molecular mechanism of adsorption 261 8.8 Conclusions and outlook 266 References 267
9. Rare earth–doped oxide materials for photoluminescence-based gas sensors 271 V. Kiisk and Raivo Jaaniso 9.1 Introduction 272 3þ 9.2 Sm :TiO2 277 3þ 9.3 Eu :ZrO2 288 viii Contents
3þ 9.4 Tb :CePO4 294 3þ 9.5 Pr :(K0.5Na0.5)NbO3 298 9.6 Conclusion 299 References 300
Part Three Methods and integration
10. Recent progress in silicon carbide field effect gas sensors 309 M. Andersson, A. Lloyd Spetz and D. Puglisi
10.1 Introduction 309 10.2 Background: transduction and sensing mechanisms 312 10.3 Sensing layer development for improved selectivity of SiC gas sensors 327 10.4 Dynamic sensor operation and advanced data evaluation 332 10.5 Applications 335 10.6 Summary 338 Acknowledgments 217 References 339
11. Semiconducting direct thermoelectric gas sensors 347 F. Rettig and R. Moos
11.1 Introduction 347 11.2 Direct thermoelectric gas sensors 353 11.3 Conclusion and future trends 380 References 381
12. Dynamic operation of semiconductor sensors 385 Andreas Schutze€ and Tilman Sauerwald 12.1 Introduction 385 12.2 Dynamic operation of metal oxide semiconductor gas sensors 388 12.3 Dynamic operation of gas-sensitive field-effect transistors 398 12.4 Conclusion and outlook 404 References 408
13. Micromachined semiconductor gas sensors 413 D. Briand and J. Courbat 13.1 Introduction 413 13.2 A brief history of semiconductors as gas-sensitive devices 414 13.3 Microhotplate concept and technologies 416 13.4 Micromachined metal oxide gas sensors 425 Contents ix
13.5 Complementary metal oxide semiconductor–compatible metal oxide gas sensors 437 13.6 Micromachined field-effect gas sensors 442 13.7 Nanostructured gas sensing layers on microhotplates 445 13.8 Semiconductor gas sensors on polymeric foil and their additive manufacturing 450 13.9 Manufacturing, products, and applications 454 13.10 Conclusion 458 References 459
14. Integrated CMOS-based sensors for gas and odor detection 465 P.K. Guha, S. Santra and J.W. Gardner
14.1 Introduction 465 14.2 Microresistive complementary metal oxide semiconductor gas sensors 467 14.3 Microcalorimetric complementary metal oxide semiconductor gas sensor 469 14.4 Sensing materials and their deposition on complementary metal oxide semiconductor gas sensors 472 14.5 Interface circuitry and its integration 475 14.6 Integrated multisensor and sensor array systems 480 14.7 Conclusion and future trends 483 Useful web addresses 485 References 486
Index 489 This page intentionally left blank Contributors
M. Andersson Linkoping€ University, Linkoping,€ Sweden N. B^arsan University of Tubingen,€ Tubingen,€ Germany D. Briand Ecole Polytechnique Fédérale de Lausanne, Neuch^atel, Switzerland Pierrick Clément Microsystems Laboratory, Ecole Polytechnique Féderale de Lausanne (EPFL), Lausanne, Switzerland E. Comini Department of Information Engineering, University of Brescia, Brescia, Italy J. Courbat Formely Ecole Polytechnique Fédérale de Lausanne, Neuch^atel, Switzerland Arnaldo D’Amico Department of Electronic Engineering, University of Rome Tor Vergata, Roma, Italy Corrado Di Natale Department of Electronic Engineering, University of Rome Tor Vergata, Roma, Italy Martin Eickhoff Institute of Solid State Physics, University of Bremen, Bremen, Germany J.W. Gardner University of Warwick, Coventry, United Kingdom P.K. Guha Indian Institute of Technology, Kharagpur, West Bengal, India Andreas Helwig Airbus Group Innovations, Munich, Germany M. Huebner University of Tubingen,€ Tubingen,€ Germany Raivo Jaaniso University of Tartu, Tartu, Estonia Teerakiat Kerdcharoen Department of Physics and NANOTEC Center of Excellence, Faculty of Science, Mahidol University, Ratchathewi, Bangkok, Thailand V. Kiisk University of Tartu, Tartu, Estonia Sung Pil Lee Kyungnam University, Changwon, Kyungnam, Korea
xi j xii Integrated CMOS-based sensors for gas and odor detectionContributors
Eduard Llobet MINOS-EMaS, Department of Electronic Engineering, Universitat Rovira i Virgili, Tarragona, Spain A. Lloyd Spetz Linkoping€ University, Linkoping,€ Sweden Konrad Maier Airbus Group Innovations, Munich, Germany R. Moos University of Bayreuth, Bayreuth, Germany Gerhard Muller€ Department of Applied Sciences and Mechatronics, Munich University of Applied Sciences, Munich, Germany D. Puglisi Linkoping€ University, Linkoping,€ Sweden F. Rettig University of Bayreuth, Bayreuth, Germany S. Santra Indian Institute of Technology, Kharagpur, West Bengal, India Tilman Sauerwald Lab for Measurement Technology, Department Systems Engineering, Saarland University, Saarbrucken,€ Germany Andreas Schutze€ Lab for Measurement Technology, Department Systems Engineering, Saarland University, Saarbrucken,€ Germany Thara Seesaard Department of Physics, Faculty of Science and Technology, Kanchanaburi Rajabhat University, Muang District, Kanchanaburi, Thailand Chowdhury Shaestagir Intel Corporation, Hillsboro, OR, United States Kengo Shimanoe Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan U. Weimar University of Tubingen,€ Tubingen,€ Germany Chatchawal Wongchoosuk Department of Physics, Faculty of Science, Kasetsart University, Chatuchak, Bangkok, Thailand Noboru Yamazoe Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan D. Zappa Department of Information Engineering, University of Brescia, Brescia, Italy PART ONE
Basics
1j This page intentionally left blank CHAPTER ONE
Fundamentals of semiconductor gas sensors
Noboru Yamazoe, Kengo Shimanoe Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan
Contents
1.1 Introduction 4 1.2 Classification of semiconductor gas sensors 5 1.3 Resistor-type sensors: empirical aspects 6 1.3.1 Sensing materials and devices 6 1.3.1.1 Sensing materials 6 1.3.1.2 Sensitizers 8 1.3.1.3 Device structure 9 1.3.1.4 Fabrication 10 1.3.2 Gas sensing characteristics 11 1.3.2.1 Response and response transients 11 1.3.2.2 Operating temperature 12 1.3.2.3 Disturbances to gas response 13 1.3.3 Semiconductor oxygen sensors 13 1.4 Resistor-type sensors: theoretical aspects 14 1.4.1 Receptor function and transducer function 14 1.4.2 Response to oxygen (base air resistance) 18 1.4.3 Response to inflammable gases 22 1.4.4 Response to oxidizing gases 23 1.4.5 Extensions 25 1.4.6 Nonresistive sensors 27 1.4.7 Field-effect transistor-type gas sensors 27 1.4.7.1 Principle 27 1.4.7.2 Solid electrolyte-gate field-effect transistor 28 1.4.7.3 Oxide semiconductor-gate field-effect transistor 29 1.4.7.4 Dielectric material-gate field-effect transistor 31 1.4.8 Oxygen concentration cell type sensors 31 1.4.9 Other gas sensors 32 1.4.9.1 Metaleinsulatoresemiconductor capacitor type sensors 32 1.4.9.2 Diode-type sensors 33
Semiconductor Gas Sensors, Second Edition ISBN: 978-0-08-102559-8 © 2020 Elsevier Ltd. https://doi.org/10.1016/B978-0-08-102559-8.00001-X All rights reserved. 3 j 4 Noboru Yamazoe and Kengo Shimanoe
1.5 Future trends 34 1.5.1 Needs and seeds 34 1.5.2 Basic approaches desired 35 1.5.3 Challenges 36 References 37
1.1 Introduction
Semiconductor gas sensors using metal oxides such as SnO2 were pioneered by two research groups in Japan.1,2 These sensors were soon put on the market as gas leak alarms and proved to be indispensable in keep- ing people safe from the distressing circumstances resulting from gas leaks. At the same time, their success had worldwide impact on researchers, creating awareness of the importance of gas sensors or chemical sensors more generally. Great effort has subsequently been made in the development of new gas sensors, including those using silicon semiconductor devices and solid electrolytes devices. If the definition of a semiconductor gas sensor is a sensor into which a semiconductor material is incorporated, there is a variety of semiconductor gas sensors of varying structures, made of different materials and involving various working principles. This introduction describes the fundamental aspects of the various semiconductor gas sensors that have been developed so far, or that are pro- posed. First, they are classified into five types, based on the constitutional principle of sensor devices (Section 1.2). The structure of devices, their working principles, and sensing mechanisms are described in subsequent sections. However, the greatest space is devoted to describing experimental knowledge and the theory of gas response of the sensors based on resistors, which have been made full use of and which still have potential for further development. It has long been queried why sensors of this type are promoted with regard to their sensitivity, as the constituent oxides are smaller than in other types of device,3 though a semiempirical analysis has been attempted.4,5 This issue was recently resolved by developing a new theory on the receptor function of small-sized oxides.6,7 As revealed in the new theory, small semiconductors are depleted of electrons in two stages by a process of ionosorption of oxygen or oxidizing gases, resulting in the appearance of regional depletion followed by volume depletion. Gas response can be sufficiently understood based on the same theory. It is shown that the theory gives an important clue to understanding the gas Fundamentals of semiconductor gas sensors 5 response of oxides attached to potentiometric gas sensors (Section 1.5). The chapter closes with personal observations regarding semiconductor gas sensors (Section 1.6).
1.2 Classification of semiconductor gas sensors Generally speaking, a gas sensor is composed of a receptor and a trans- ducer, as illustrated in Fig. 1.1. The former is provided with a material or a materials system which, on interacting with a target gas, either induces a change in its own properties (work function, dielectric constant, electrode potential, mass, etc.) or emits heat or light. The transducer is a device to transform such an effect into an electrical signal (sensor response). The construction of a sensor is determined by the transducer used, with the receptor appearing to be implanted within it. From this perspective, a semi- conductor gas sensor can be defined as a sensor in which a semiconductor material is used as a receptor and/or transducer. There are two groups of semiconductors: oxide and nonoxide (typically, silicon). Nonoxide semiconductors cannot work as a receptor because they are coated with a protective insulation layer, but they can provide a trans- ducer in the form of MIS FETs (metaleinsulatoresemiconductor field- effect transistor) and MIS capacitors. In contrast, oxide semiconductors can work as both a receptor and a transducer (mostly in the form of a resistor)
Figure 1.1 Gas sensor as constituted of a receptor and a transducer. R ¼ resistance,
E ¼ electromotive force, I ¼ current, Vth ¼ threshold voltage (FET), Cp ¼ capacitance. 6 Noboru Yamazoe and Kengo Shimanoe owing to their chemical and physical stability in hostile environments at elevated temperatures. Table 1.1 shows various examples of semiconductor gas sensors classified according to the types of transducer used and subclassified by the kinds of receptor used, together with the kinds of signal output (response), typical sensor devices, and the gases targeted. The transducers are seen to be available in the forms of resistors, diodes, MIS capacitors, MIS FETs, or oxygen concentration cells. For each type of sensor thus classified, devices, sensing principles, and the important features of semiconductor gas sensors are now described.
1.3 Resistor-type sensors: empirical aspects Of the various types of sensor, resistor sensors have received the great- est investigation and have proven their feasibility in practice. These sensors are often called “oxide semiconductor gas sensors.” There are two subtypes: surface sensitive and bulk sensitive. This section is devoted to surface- sensitive resistor sensors, except for Section 1.3.3 which briefly discusses bulk sensitive resistor sensors. It is noted that books and review articles e have been published about oxide semiconductor gas sensors.8 10 1.3.1 Sensing materials and devices 1.3.1.1 Sensing materials A surface-sensitive resistor sensor works on a very simple principle; on exposure to a target gas in air at an elevated temperature, its resistance either decreases or increases as a function of the partial pressure of the gas. Of the many metal oxides, n-type oxides (SnO2,In2O3,WO3, ZnO, and g-Fe2O3) and p-type oxides (CuO and Co3O4) exhibit significant gas sensing properties. Mainly because of stability issues, however, SnO2,In2O3, and WO3 have been adopted as the sensor materials utilized in practice. In practice, even these oxides are frequently loaded or mixed empirically with several foreign materials as a sensitizer (PdO, Pt, Fe2O3, etc), a skeleton material (alumina), or a binder (silica). When an n-type oxide is used, resistance decreases on exposure to inflammable or reducing gases in the air (inorganic: H2, CO, NH3,H2S, NO, etc; organic: CH4, propane, alcohols, odorants, etc.), while it increases on exposure to oxidative gases (NO2, ozone, N2O, etc.). Apart from such redox-active gases, CO2 and water vapor have been known to affect the resistance to a greater or lesser degree. Exploitation of the effects of CO2 11 has led to the development of a semiconductor CO2 sensor. udmnaso eiodco a sensors gas semiconductor of Fundamentals
Table 1.1 Classification of semiconductor gas sensors according to the types of transducers and receptors used. Transducer Response signal Receptor Device (example) Target
Resistor Resistance Oxides Porous SnO2 (surface- A variety of gases sensitive) Sintered TiO2 (bulk- Air/fuel ratio (car sensitive) engine) Diode Bias current Oxides Pd-TiO2 (single crystal) H2 Metaleinsulator Bias potential shift Pd Pd-gate capacitor H2,NH3 esemiconductor (MIS) capacitor MIS field-effect Threshold voltage shift Pd Pd-gate FET H2,NH3 transistor (FET) Ionic Proton H2 conductors conductor gate FET NaNO2-gate FET NO2 Oxides WO3-gate FET NO2 Dielectrics Cellulose-gate FET Humidity Oxygen concentration Cell voltage Oxides Pt/zirconia/oxide/Pt A variety of gases cell
Note: “Oxides” stands for semiconductive metal oxides. 7 8 Noboru Yamazoe and Kengo Shimanoe
1.3.1.2 Sensitizers Gas sensing properties, especially gas responses, are known to be often improved significantly when constituent oxides are loaded with small amounts of appropriately chosen foreign materials. Examples are SnO2- PdO (CO, propane, etc.), SnO2-Pt and/or PdO (methane), SnO2-Co3O4 (CO), SnO2-CuO (H2S), SnO2-Ag2O(H2), In2O3-PdO (CO, odorant gases), WO3-Au (NH3), SnO2-La2O3-Pt (ethanol), SnO2-CaO (ethanol), In2O3-Fe2O3 (ozone), SnO2-Fe2O3 (NO2), TiO2-Cr2O3 (NO), etc. In this list, the materials following the oxide semiconductors are sensitizers and the target gases are shown in parentheses. As suggested from the large variation in sensitizers, the mechanisms of sensitization involved are not so simple. It is useful to know that the dispersion of the sensitizers, except Pt, al- ways causes the resistances of the device in base air to increase. This suggests that those interact with the oxides and increase the work function of the oxides. In view of heterogeneous catalysis, Pt, PdO, CuO, Ag2O, Co3O4, and Au are well-known oxidation catalysts to reducing gases. Therefore, such catalytic activity is relevant to the sensitizing actions. It should be noted, however, that the mere promotion of oxidation reactions cannot contribute to gas response unless it has something to do with the surface properties of the oxides. In this sense, the sensitizers, except Pt, undergo redox changes such as PdO þ H2 / Pd þ H2O, Pd þ (1/2) O2 / PdO, and the changes of their redox state on exposure to target gases can possibly induce changes in device resistance (gas response) through electronic interactions with oxides (electronic sensitization). In the case of Pt, on the other hand, it seems that the target gas (methane) is partially oxidized on Pt to HCHO or CO, which then reacts with the adsorbed oxygen of the oxide (chemical sensitization). La2O3 and CaO, which have no such catalytic oxidation activity, modify the acid-base properties of the oxide surface more basic; on the acidic surface, ethanol undergoes dehydration (no consumption of O ), C2H5OH / C2H4 þ H2O; on the basic surface, it undergoes oxidative dehydrogenation, C2H5OH þ 2O / C2H4O þ H2O. It is thus under- stood that, in this case, the selectivity of reaction paths is changed by the sensitizers. As shown above, Fe2O3 promotes response to oxidizing gases, though the mechanism of promotion is not yet clear. There can be no doubt that sensitizers are very important for practical devices. Unfortunately, however, little basic research has been carried out on sensitizers and sensitizing actions. Fundamentals of semiconductor gas sensors 9
1.3.1.3 Device structure Sensor devices are fabricated into a resistor in which a porous stack of the sensing materials is attached with a heater and a resistance measuring probe (usually a pair of metal electrodes). Various structures have been devised in practice, as shown in Fig. 1.2. Originally fabrication was a sintered block structure (about 0.5 cm in size) with a pair of Pt coil electrodes inserted (a); one of the coils also served as a heater. This was followed by a thin alumina tube within a heavy coating (b); a pair of wire electrodes was wound on the tube and a heater was set inside it. Currently in wide use is a thick film structure (c), screen-printed on an alumina substrate with a pair of elec- trodes, and a heater printed in advance. A microversion of this structure,
Figure 1.2 Device structures adopted for resistor-type sensors in practice. (a) Sintered block, (b) thin alumina tube-coated layer, (c) screen printed thick film, (d) small bead inserted with coil and needle electrodes, (e) small bead inserted with a single coil (heater and electrode), (f) practical sensor element assembling sensor device, metal cap, and filter. 10 Noboru Yamazoe and Kengo Shimanoe known as a MEMS (microelectromechanical system) sensor, is currently under development, as will be described later. Apart from these standard structures, bead-shaped structures have been devised for practical use. A small bead made of sensing materials (about 0.5 mm in size) is inserted with a coil and needle electrodes in (d); the coil also works as a heater. A similar bead is inserted with a single coil (heater) in (e), the so-called “hot wire” type; a change in the resistance of the sensing materials affects the composite resistance between the two terminals of the inserted coil, which is measured precisely on a bridge circuit as gas response. For actual use, each device is bonded to the connector pins and put inside a metal cap with a hole(s) on top to remove the risk of triggering gas explosions. In addition, an adsorbent such as active carbon (often referred to as a “filter”) is placed in a layer immediately behind the hole to remove unpleasant gases, as shown in (f).
1.3.1.4 Fabrication Important guidelines for device fabrication collected empirically can be summarized as follows: 1. Crystallite sizes of oxide semiconductors should be as small as possible. 2. Sensitizers should be dispersed as finely as possible. 3. Sensing layer thickness and porosity should also be optimized to improve selectivity and durability. According to these guidelines, fabrication of devices is carried out care- fully. It starts with the preparation of a fine powder of oxide semiconductor (crystallite size around 10 nm in diameter) through what is known as a “wet” process. This is the precipitation of a precursor of the oxide from an aqueous solution of its metal salt(s), followed by the gentle washing, drying, and calcination of the precursor before its conversion to the final powder. The powder is loaded with a small amount of a sensitizer and then converted into slurry (paste) by milling it using water or organic vehicles, together with any other necessary additives. The slurry is finally deposited over the electrodes (block or bead type) or on the substrate (thick film type), and, after drying, the deposit is sintered under specific conditions to stabilize the porous microstructure. It is noted that all of the semiconductor gas sensors so far in use are of the thick film (or layer) type, prepared through the wet processes discussed above. Thin film type devices, especially those fabricated via physical methods such as sputtering, have frequently shown interesting sensing per- formances in the short term, but little use is currently made of these devices. Fundamentals of semiconductor gas sensors 11
1.3.2 Gas sensing characteristics 1.3.2.1 Response and response transients The behavior of resistance on switching between base air and gas ambient is illustrated in Fig. 1.3(a). On switching to an inflammable gas ambient, the resistance reduces from a value in air (Ra) to a stationary value (Rg), while it goes back to Ra on switching back. Empirically, gas response is defined as the ratio Ra/Rg (normalized conductance). The rate of response or recovery is expressed empirically in terms of the time (s) needed for a 90% full response or recovery. In the case of oxidizing gases such as NO2, which increase the resistance, gas response is defined as Rg/Ra (normalized resistance). The dependence of Rg on the partial pressure of target gas (Pg)is known empirically to fall on linear correlations on logarithmic scales12; ¼ a that is, Rg cPg , where a and c are constants (power law), as shown in Fig. 1.3(b). Accordingly, gas response also follows power law, = ¼ a fl = ¼ a Ra Rg cPg (in ammable gases) or Rg Ra cPg (oxidizing gases).
Figure 1.3 Response and recovery transients. (a) On switching on and off an inflam- mable gas in air, (b) linear correlation observed between resistance (Rg) and partial pressure of the gas (Pg) on logarithmic scales (power law). 12 Noboru Yamazoe and Kengo Shimanoe
The power index, a, is almost fixed depending on the kinds of target gas, taking values roughly equal to 1/2 to many inflammable gases (H2, CO, etc), 1 to NO2, and 1/2 for O3. It is noted that the resistance under exposure to varying partial pressure of oxygen (PO2) follows the power equation with = a ¼ 1=2, namely, R ¼ c'''P1 2. The power indices are related to the O2 O2 modes of interaction between the gases and the surface of oxide semi- conductors, as will be discussed later. Sensitivity is usually defined as a slope of the correlation between gas response and Pg. In the event that power law holds well, however, this definition is meaningless because sensitivity is dependent on Pg unless a fi a is unity. This dif culty is overcome if Pg is replaced by Pg in the above definition. The slope (sensitivity) is then nothing but the proportionality constant of the power equation. Sensitivity is determined by the physico- chemical constants of semiconductor, target gas, and oxygen. 1.3.2.2 Operating temperature Response and response transients are sensitive to the operating temperature. The rates of response and recovery naturally increase with increasing tem- perature. On the other hand, response shows different behavior depending on whether the gas is inflammable or oxidizing. For an inflammable gas, response goes through a maximum on increasing temperature, resulting in a well-known bell-shaped correlation between the response and temper- ature. This dependence appears because the rate constant of the surface reaction between gas and adsorbed oxygen (kR) increases exponentially with a rise in temperature, while the Knudsen diffusion coefficient of the gas (DK) does so sublinearly. In the lower temperature region, kR < DK is held so that kR is an exclusive determinant for gas response. In higher temperatures, on the other hand, the relation is inversed, kR > DK, and the response is attenuated by the gas diffusion and reaction effect.13,14 In this temperature range, the gas is consumed significantly by diffusion from the surface to the inside of the porous sensing layer. The effective partial pressure of the gas in the inner region where the resistance is actually measured can be significantly lower than the nominal value outside. The ratio of the actual gas response to the ideal (free of attenuation) is known as the “utility factor” (U). U remains unity in lower temperatures, while in higher temperatures it decreases rather sharply with increasing tempera- ture, increasing diffusion length (sensing layer thickness), and decreasing pore size. It follows that the response maximum and the temperature at that point vary not only by the kinds of gas and oxide semiconductor Fundamentals of semiconductor gas sensors 13 but also by the device structure (layer thickness, in particular) and the sensing materials adopted in processing. Strictly speaking, there is a further possible reason for the decrease of gas response at high temperature: oxygen adsorption is decreased with increasing temperature. Therefore, if the partial pressure of the inflammable gas is too great, adsorbed oxygen is consumed (resistance reaches minimum) such that gas response will decrease with increasing temperature, reflecting the temperature dependence of the adsorbed oxygen. This discussion is valid for a small partial pressure of gas. Oxidizing gases such as NO2, on the other hand, are adsorbed on the oxide semiconductor particles. The amount of adsorption, and therefore the gas response, increases as the temperature drops. Operating temperature is then determined as a compromise between gas response and rates of response and recovery.
1.3.2.3 Disturbances to gas response Gas response reacts to disturbances to varying degrees. There are two kinds of disturbance: a drift of base air resistance (Ra) and a modulation of gas response (Rg) by coexistent gases. As for the former, Ra shifts downward quickly on increasing the partial pressure of coexistent water vapor “ ” (PH2O), a phenomenon known as a short-term effect of water vapor.
Apart from this phenomenon, PH2O seems to be related to a long-term drift of Ra; it is known that Ra undergoes seasonal changes; that is, it goes up in summer and goes down in winter. Unfortunately, these two types of drifts are yet to be clarified in detail. Practically, attempts have been made to correct the long-term drift partly by means of software. The disturbance brought about by a modulation of gas response can be simplified if both the target gas and coexistent gas are inflammable, as in the case of sensing CO in the coexistence of H2. The strength of the disturbance can be estimated if the sensitivity to each gas is known. To mitigate interfer- ences by coexistent gases, nonstandard modes of sensor operation have been adopted in some cases for sensing CO and alcohol in the breath. 1.3.3 Semiconductor oxygen sensors At sufficiently high temperatures, where the bulk diffusion of component ion oxides is activated to a significant degree, oxide semiconductors are known to change nonstoichiometry, and thus electronic conductivity fl changes depending on PO2. On exposure to a mixture of in ammable gas and air, sensors using such oxides change resistance depending on the composition of the mixture. What is responsible for the change in resistance 14 Noboru Yamazoe and Kengo Shimanoe
is not the reducing gas itself but PO2 in the ambient after the reducing gas has been oxidized completely. Resistor-type oxygen sensors working on this principle have been proposed by using oxides such as TiO2,Nb2O5, and MgO-CoO. Among them, one using TiO2 has been successfully incorpo- rated into car engine exhaust control systems in practice. The sensor, fabricated into a well-sintered block of TiO2 with a pair of electrodes inserted, is exposed to car engine exhausts at high temperature (e.g., 1073K). As the resistance decreases or increases stepwise as air/fuel (A/F) ratio crosses the border between lean burn and rich burn, it can be utilized for A/F ratio control. Its share in the market is somewhat small, however, compared with that of its competitor, zirconia oxygen sensors.
1.4 Resistor-type sensors: theoretical aspects For resistive-type gas sensors, a porous assembly of fine particles (mostly grains) of oxide semiconductors should function as a receptor and a transducer. It has long been accepted that grains act as a receptor to gases, while the contacts between the grains act as the transducer which transforms the gas reception into a change in device resistance. However, an under- standing of the receptor function and the transducer function involved had remained far from being satisfactory until basic approaches to them began very recently. This section focuses on recent advances in the basic (theoretical) approaches, though the studies are still in progress. 1.4.1 Receptor function and transducer function Oxide semiconductors are known to exhibit unique interactions with some sorts of gases, resulting in the ionosorption of the gases. In the event that the gas in a problem situation has a large electron affinity, such as O2 and NO2, the host semiconductor supplies electrons to the gas to allow it to be 2 adsorbed as anionic species such as O ,O or NO2 . In the event that the gas is low in ionization potential, such as NO, on the other hand, the gas donates electrons to the semiconductor to be adsorbed as cationic spe- þ cies, such as NO . The electrons supplied or given up in these ionosorption processes are transferred from the bulk of the semiconductor to the surface, or vice versa, accompanied by a change in energy band structure (band bending) of the semiconductor. It is well-known that electron transfer from the bulk of n-type semiconductor results in the formation of an electron-depleted layer in the semiconductor. No doubt, an oxide semicon- ductor sensor, when placed in air, is subjected to the adsorption Fundamentals of semiconductor gas sensors 15
(ionosorption) of oxygen, and its resistance in air (air base) is determined usually from the equilibrium of oxygen adsorption. As very recently revealed with SnO2 sensors, oxygen is adsorbed mainly in the form of O2 in extremely dry air, whereas in the presence of low humidity (0.1% in volume and above), the adsorption in that form is suppressed almost completely by water vapor to be replaced by the adsorption in another form (O ). In practice, it can thus be assumed as a good approximation that the latter form (O ) prevails over the former (O2 ) under usual sensor operating conditions. The sensor is utilized for detecting a target gas coexistent in air by means of a change in the resistance of the device. Target gases fall into two groups: gases which undergo ionosorption (such as NO2) and inflammable gases (such as H2, CO, and C3H8). In cases where ionosorption takes place in addition to that of oxygen, the energy band structure changes accordingly. Usually, however, serious interference often occurs between the ionosorption of the gas and that of oxygen, reducing the resultant change in energy band structure. The key to designing a sensor sensitive to such a gas is discovering how to mitigate such interference. Inflammable gases react with the anionic adsorbates of oxygen. As a result of the reaction, electrons of the adsorbates are returned to the semi- conductor, causing the energy band structure to revert to one that corresponds to smaller amounts of oxygen adsorbates. Obviously, response to a gas in this group will be enhanced as the consumption of the oxygen adsorbates is made more efficient. Here, it is of central importance to show how the qualitative understand- ing mentioned above can be converted into more quantitative ones. For simplicity, let us assume that a sensor device is a porous stack of uniform grains of an n-type oxide semiconductor. It is accepted that each grain plays the role of a receptor, while that of the transducer is played by each contact between grains; that is the most resistive part in the device, so it determines the resistance of the whole device. However, further understanding has been less than straightforward. For some considerable time, efforts were made to understand the receptor and the transducer functions based on the surface space charge layer model and the double Schottky barrier model, as shown by (a) and (b) in Fig. 1.4, respectively. These models, (a) and (b), were guessed at by many researchers as analogies from a metal semiconductor contact diode (see, for instance, Ref. 9). It was assumed that the thickness of the depletion layer (w) should increase as oxygen adsorption as anionic species (typically O ) increases, while it should decrease as the adsorbed oxygen is consumed with an inflammable gas (H2). Correspondingly, the 16 Noboru Yamazoe and Kengo Shimanoe
Figure 1.4 Diagrams of electron depletion for oxide grains and the resistance of contact between grains. (a) Space charge layer model, (b) double Schottky barrier model, (c) regional and volume depletion model, (d) surface conductive grains contact model. double Schottky barrier formed across the contact between grains should change its height, inducing changes in contact resistance and, hence, resistance in the device. Unfortunately, these models were unable to give quantitative information regarding gas response. Shortcomings of the models were made clear recently by our basic approaches, as described below. The receptor model (a) assumes implicitly that the semiconductor grains are sufficiently large. In reality, however, they are very tiny (typically about 10 nm in diameter), so the space charge layer can easily extend over the fi entire area of grains; that is, w grows to grain radius (a), at PO2 signi cantly below that in air, PO2 (a). Obviously, a new process of electron depletion has to take place afterward until the grains reach electrostatic equilibrium with oxygen adsorption at PO2(a). A method proposed here is one in which electron depletion is achieved by shifting the Fermi level downward by p kT, as shown in Fig. 1.5.6,7 Here, p is the Fermi level shift as expressed in the unit of kT, where kT is thermal energy. The electrons supplied to the adsorbates in this stage are squeezed out of the grains by increasing p.To distinguish the electron depletion of this type (accompanied by a change in p) from the conventional type one (accompanied by a change in w), these are denoted as volume depletion and regional depletion, respectively. The value of p or w is determined uniquely for given conditions of gas adsorption and semiconductor grains. Importantly, p or w depends on a when the Fundamentals of semiconductor gas sensors 17
(a) (b) PO2 = 0 Ec Ec PO2(I) PO2(I) O–(I) O–(I)
p(II)kT) PO2(II)
– qV(r) O (II) qV(r) PO2(II) p(III)kT) O–(II) p P (III) (III)kT) O–(III) O2 P (III) O–(III) O2 –a 0 a –a/2 0 a/2 r r (c) (d)
Nd Nd P (I) PO2(I) O2 n PO2(II) n
PO2(II)
PO2(III)
P (III) 0 O2 0 –a 0 a –a/2 0 a/2 r r Figure 1.5 Energy band diagrams: (a) and (b) distributions of conduction electrons; (c) and (d) for two kinds of grains different in radius (a or a/2) at steps of increasing . PO2 conditions are otherwise fixed. As shown in Fig. 1.4(c), small oxides are ; usually in a state of regional depletion at low PO2 while those that are usually in a state of volume depletion in base air (the whole area being depleted) and their electronic states are controlled by p. It is noted, however, that more rigorous discussion should be extended in terms of reduced radius (n) rather than of radius (a), as discussed later. The double Schottky barrier model (Fig. 1.4(b)) also turned out to be completely misleading. It focused attention on the electron transport path running through the centers of contacting grains. In reality, however, there are a tremendous number of other transport paths running on the surface of grains, which are free of potential barriers, as shown in Fig. 1.4(d). The electron transport through the contact can thus be achieved by migration or tunneling of the surface electrons, indifferent to the bulk electrons inside. The contact resistance and the device resistance (R) are then inversely proportional to the surface density of electrons, [e]S, as long as the grains are uniform. Device resistance (R) as normalized by that at flat band state 18 Noboru Yamazoe and Kengo Shimanoe
(R0), called “reduced resistance,” is expressed by using the donor density of semiconductor (ND) as follows: R ¼ ND e S (1.1) R0 1.4.2 Response to oxygen (base air resistance) Let us consider a case where oxygen is adsorbed as O on an oxide grain of radius a. The adsorption equilibrium is written as follows: þ ¼ 0 2 ¼ 2 O2 2e 2O KO2PO2 e S O (1.2) Here, KO is the adsorption constant and [O ] the surface concentration of 2 O . Note that [e]S is a variable of the grain. At the same time, we have to consider the electrostatic equilibrium of the grain. Assuming that there is no surface state other than O ,[e ]S and [O ] can be expressed as a function of p, respectively, for volume depletion as follows:15 n o Q n O ¼ SC ¼ N L AðnÞexpð pÞ (1.3) q D D 3 1 ½e ¼ N exp n2 p (1.4) S D 6
Here, QSC is the total surface charge density of the grain, which is assumed to be ascribed solely to [O ] in this case. q is the elementary charge 2 1/2 of proton. LD is the Debye length defined as LD ¼ (εkT/q ND) , where ε is permittivity, and n is reduced radius defined as n ¼ a/LD. A(n) stands for the number of free electrons remaining in the conduction band at p ¼ 0as normalized by NDLD and the surface area of the grain. Assuming Boltz- mann’s distribution law for the tailing of electrons, it is given by the following integral: Z n ð Þ¼ 1 2 1 2 A n 2 R exp R dR n o 6 There are three simultaneous equations, Eqs. (1.2)e(1.4), correlating among three variables, [e ]S,[O ], and p. It is thus possible to determine each variable as a function of KO2PO2. The solution for [e ]S is transformed into normalized resistance through Eq. (1.1). N R S = D ¼ ¼ cðnÞþ ðK P Þ1 2 (1.5) ½ O2 O2 e S R0 a Fundamentals of semiconductor gas sensors 19
S is the shape factor for the semiconductor crystals used; i.e., S ¼ 3 for spheres, 2 for columns, and 1 for plates. Constant c(n) is given by c(n) ¼ (3/n) exp (n2/6) A(n); it increases from unity as n increases; first, gradually when n is small and then exponentially afterward. The correlations given by Eq. (1.5) are illustrated in Fig. 1.6, where ð Þ1=2 reduced resistance (R/R0) is related to KO2 PO2 for variously sized grains (LD is assumed to be 3 nm). The linear correlations coincide with the power index (1/2) to PO2, as previously mentioned. Its slope is given by (3/a), indicating that sensitivity to oxygen increases as a decreases. As also indicated in Fig. 1.6, the correlation is bent in the initial region of
PO2 for larger grains where regional depletion takes place. Remarkably, it can be shown that R/R0 is almost independent of a in the regional area. Such correlations have, in fact, been confirmed experimentally. It is also noted that, under a particular condition, oxygen adsorption to form another species (O2 ) also takes place, which is demonstrated by the linear = dependence of R/R on P1 4 in the stage of volume depletion. 0 O2 Notably, the sensor response is related to the kind and amount of oxygen species adsorbed on the surface of the metal oxide semiconductors. The adsorption equilibrium for O and O2 can be discussed as follows, respectively.